Parasympathetic nervous system

The parasympathetic nervous system (PSNS) is a vital division of the autonomic nervous system. The autonomic nervous system manages the body's unconscious actions. This includes the enteric nervous system and the sympathetic nervous system. While the sympathetic system triggers the "fight-or-flight" response during stress, the PSNS promotes "rest-and-digest" activities. It helps the body conserve energy and store nutrients through metabolic processes like glycogen synthesis. By managing these functions, the PSNS maintains internal homeostasis over long periods of time.

To send signals, the PSNS uses a pathway involving two specific neurons. The first is the preganglionic or presynaptic neuron. Its cell body is located in the central nervous system (CNS). The axon of this neuron is usually long. It extends from the CNS to a cluster of cells called a ganglion. This ganglion is located either very close to or inside the target organ. The second neuron is the postganglionic or postsynaptic neuron. Its cell body sits within the ganglion. Because the ganglion is near the organ, these second fibers are very short. Unlike the discrete connections in the somatic nervous system, these fibers often release neurotransmitters across a wide area of smooth muscle and glands.

The PSNS is characterized by its "craniosacral outflow." This means the nerve fibers arise from the cranium and the sacrum of the spine. In the cranium, several cranial nerves carry these signals. These include the oculomotor, facial, glossopharyngeal, and vagus nerves. In the lower spine, the pelvic splanchnic nerves arise from the sacral segments S2 through S4. These nerves exit the vertebral column via the anterior rami. They travel to synapse at autonomic ganglia located near the walls of pelvic organs. This structure differs from the sympathetic system, which has a "thoracolumbar outflow."

Specific cranial nerves manage different parts of the body. The oculomotor nerve controls the eye through the ciliary ganglion. It manages the ciliary muscle for accommodation and the iris sphincter for miosis, which is pupil constriction. The facial nerve regulates secretions for the lacrimal gland to produce tears. It also controls the sublingual and submandibular salivary glands via the chorda tympani nerve. The glossopharyngeal nerve manages the parotid salivary gland. This process involves the tympanic plexus and the otic ganglion. Each of these nerves ensures that specific glands and muscles respond correctly to the body's needs.

The vagus nerve is perhaps the most extensive part of the system. Its name comes from the Latin word for "wandering." This describes how it reaches a massive array of organs. It travels through the neck and into the thorax. There, it forms the esophageal plexus with sympathetic nerves. The vagus nerve provides innervation to the heart, lungs, and esophagus. It also travels into the abdomen to reach the pancreas, liver, kidneys, and stomach. Its reach extends down the gut tube until the splenic flexure of the transverse colon.

The vagus nerve also plays a role in the body's immune response. It is part of an "inflammatory reflex." In this process, efferent vagal fibers use cholinergic signaling to act on the body. This action helps dampen the release of pro-inflammatory cytokines. By doing this, the nerve modulates innate immune responses. Researchers can study this activity by looking at heart rate variability (HRV). This measure reflects how the vagus nerve modulates the sinoatrial node of the heart. It serves as a surrogate marker for cardiac parasympathetic tone in clinical settings.

The complexity of the PSNS allows for highly coordinated biological functions. For example, the recurrent laryngeal nerve is a large branch of the vagus nerve. On the left side, it hooks around the aorta. On the right side, it hooks around the right subclavian artery. These unique paths are results of how the circulatory system developed during embryology. These nerves supply the larynx, trachea, and esophagus. By connecting different systems, the PSNS ensures that digestion, immunity, and heart function work together. This creates a stable environment for the body to recover and grow.